The present invention relates to loudspeakers and, in particular, to loudspeakers having low distortion and excellent transient response.
Recent advances in solid-state technology have resulted invastly improved tuners, turntables, tape decks, and amplifiers for use in music-reproduction systems. These improved units have required attendant corresponding improvements in loudspeakers. One of the principal requirements of modern-day loudspeakers is that they be able to handle the large amounts of audio power necessary to produce the desired sound-pressure levels. In addition to this power-handling requirement, there is the requirement that the output of the loudspeaker be a faithful reproduction of the signals originating from the music source. These system requirements pose a particularly difficult problem in the design and manufacture of low-frequency loudspeakers, i.e., woofers.
It is known that when a loudspeaker operates in the flat region of its response curve, the volume of air it must move quadruples for every octave reduction in frequency. The effect of this is that at 30 Hz a radiator or loudspeaker must vibrate with an excursion over one thousand times greater than at 1 kHz, in order to produce the same acoustic pressures. It has been shown that a 10 inch driver, having an effective radiating diameter of 20 cm and operating at 30 Hz, must move linearly over a distance of 1.5 cm, in order to produce a sound-pressure level of 105 dB at a distance of 1 meter.
Various approaches to increasing the accuracy, efficiency, and response of loudspeakers have involved employing specialized cone materials and providing specialized voice-coil arrangements, so that during maximum excursion of the voice coil, there is no opportunity for the voice coil to bottom out. The problem meets the solution head on, since one method of making a high-fidelity speaker, which can produce low frequencies with low distortion, is to use a lengthened voice coil. Obviously, a lengthened voice coil will have a greater tendency to bottom out during maximum excursion and, thereby, will have a greater tendency to clip the signal and limit the high-fidelity capabilities of the speaker. Also, it is known to produce inexpensive speakers, having high-power handling capabilities, by using a short voice coil.
Additionally, a type of dynamic drive force distortion, which can be termed "motional induction" comes into play in loudspeaker operation and has the effect of increasing the distortion and reducing the power-handling capability of a loudspeaker, by moving the effective rest point of the driver back and forth, in relation to the amplitude and frequency of the signal it is producing. In other words, because the rest position in the operating state is not at the initially designed point, the effective cone excursion is reduced, thereby causing the above-mentioned problems. Moreover, even-order distortion components are also introduced by motional inductance. Finally, second-order distortion components will be produced by the changing permeation of the magnetic field, caused by the cone and voice coil combination moving back and forth during normal operation.
Another problem associated with motional inductance is that the reactive properties of the driver or loudspeaker change instantaneously with the change in voice-coil position. It is essentially impossible to compensate for these changes in reactive properties using passive electronic components.
As indicated above, in the past, the bottoming-out problem has been overcome by the use of a relatively short voice coil. In this regard, it has been proposed to focus the magnetic field in the voice coil vicinity by chamfering the top portion of the center pole. It has also been proposed to provide an undercutting of the center pole, to form a crown at the top of the center pole. This undercutting is done in order to permit a small amount of misalignment between the voice coil and the center pole, which will prevent binding if such misalignment is present. Such misalignment permits the manufacturing tolerances to be relaxed, making assembly easier and less costly. An interesting discussion of minimizing flux leakage in the magnetic gap and minimizing the influence of axial nonuniformity of the magnetic field in the air gap is found in Loudspeakers, N. W. McLachlan, Dover Pub., pp 239-241, (1960).